NEWPORT NEWS — As Northrop Grumman Corp. on Monday continued its investigation into dozens of employees at its Newport News shipyard on suspicion of illegal drug use and possession, the yard's general manager asked employees to stay focused and warned that "a lapse in judgment can be devastating."

In an e-mail message sent Monday morning to all of Northrop's Newport News employees, Shipbuilding Vice President and yard General Manager Matt Mulherin repeated the company's policy not to comment publicly on what it considers a "personnel matter," and said he "cannot address the information" contained in a Saturday Daily Press report detailing the investigation.

A company spokeswoman also declined to comment.

In his four-paragraph message, Mulherin said he wanted to "take this opportunity to share my thoughts about the responsibility we all have as shipbuilders."

"Being a shipbuilder means more than just fitting pipe, performing engineering calculations, measuring tolerances, bending steel, welding, pulling cable and all the other daily jobs we perform. It means working safely, providing the highest standards of quality and acting with integrity, always," Mulherin wrote.

"I'm asking you to stay focused — to remember the work we do directly contributes to the national security of our country. A lapse in judgment can be devastating, not only for an individual employee or our company, but for the American heroes who serve on our ships," the letter said.

Meanwhile Monday, several sources told the Daily Press that the company's investigation into as many as 40 workers continues. Several remain on administrative leave pending the results of drug tests, while others will remain off the job until the investigation concludes, said two sources with knowledge of the investigation.

The probe was launched on Feb. 23 and includes hourly and salaried workers, including more than a dozen student workers affiliated with the company's trade school, The Apprentice School, sources said.

At least two of those students were banned from participating in the school's Saturday graduation ceremony, when 161 were scheduled to graduate. A company release on Monday said only that "over 150 students" participated.

As of Friday, company investigators had questioned several workers suspected of drug use and searched lockers, toolboxes and employees' personal vehicles parked on shipyard property. At least 13 workers were required to submit to drug tests and inspectors found marijuana in at least two vehicles parked on shipyard property, sources said.